Space launch vehicles typically use cryogenic propellants, such as liquid hydrogen and liquid oxygen due to the high efficiency provided in terms of thrust-to-weight ratios. In the main engine of typical hydrogen-fueled rockets, liquid hydrogen is heated, vaporized, and fed to the main combustion chamber by a fuel turbopump. Liquid oxygen is likewise fed to the main combustion chamber by an oxidizer turbopump which may or may not be driven by the same shaft as the fuel turbopump. The power to run the fuel and oxidizer turbopumps is supplied by a turbine which is driven by the high pressure hydrogen gas that has been vaporized in a heat exchanger which surrounds the main combustion chamber. The fuel and oxidizer turbopumps thereby provide the necessary flow rates of fuel and oxidizer needed to produce the desired thrust in the main rocket engine.
Because turbopumps add greatly to the cost and complexity of a rocket engine, some rocket engines simply utilize a pressurized gas source such as helium to pressurize the liquid hydrogen and liquid oxygen tanks. The flow of hydrogen and oxygen to the engine can then be controlled by incorporating a valve into each of the flow lines which connect the combustion chamber to the hydrogen tank or oxygen tank. This latter design is commonly used on booster engines since it avoids the need for costly turbopumps. The drawback of the helium pressurized rocket system is the significant weight penalty incurred through the use of the large, heavy helium tank.
For an earth-to-orbit space launch vehicle, several helium pressurized booster rockets may be "strapped" to a turbopump pressurized main rocket engine. The sum of the weight penalties incurred by each of the helium pressurized booster rockets is substantial as compared to the payload which can be lifted by the space launch vehicle. If the need for the helium tanks in each of the booster engines could be eliminated, a greater payload could be carried by the space launch vehicle.
What is needed is a space launch system in which the booster rockets' tanks of helium can be eliminated without incurring the cost and complexity of incorporating turbopumps into the booster rockets.